To improve performance, some window manufacturers in North America fill their insulating glass windows with special gases. Typically, sulphur hexafluoride (SF6) is used where sound insulation is important and argon is used when thermal insulation is important. The benefits of thermal insulation are gaining universal acceptance by window manufacturers and consumers alike, since thermal performance enhancements of 10 to 25 percent can be achieved with an argon fill. On a national scale, this improvement is significant.
The present invention is concerned with the problem of monitoring the gas content of insulating glass units. Gas content measurements are desirable for several reasons: quality control at production, quality assurance at the construction site before and after installation, long term field tests, accelerated weathering studies, and so forth. Heretofore, the gas content of a window unit has been determined by puncturing the gas seal of the window unit to remove a sample of gas for analysis. The repair of the puncture is not necessarily as secure or reliable as the overall construction of the window unit and, accordingly, a better product can be obtained if a non-intrusive test was available for both quality control and long-term field-performance tests.
A known non-intrusive measurement technique involves focusing monochromatic light onto the gas in the window unit and observing the scattered light signal. Using the known Raman scattering phenomenon, the wavelength for resonant scattering off of either the oxygen or nitrogen molecules, a well defined signature of the presence of either gas, can be used to determine the quantity of air in the unit. With this technique, a precision of about 0.2 percent can be achieved after five scans. This technique does not measure argon directly. It must be calculated from the measured air content. Further drawbacks of this technique are that it requires vibration isolation, an optical table with expensive state of the art technology, a high level of technical expertise and it can only be used in laboratory applications at present. Accordingly, it is not a viable solution.
Another method of determining the gas content of a window unit is to measure its thermal conductance. This method provides a qualitative indication of whether the window unit is above 75 percent filled with argon and provides a quantitative indication if its fill level is below 75 percent. However, the method is slow and, as indicated, does not provide precise numbers above 75 percent argon content. Manufacturers achieve a 95-100 percent argon fill so that a typical leakage of less than 0.5 percent argon loss per year guarantees at least a 20 year life span for the unit before the argon content drops below 75 percent and thermal performance begins to deteriorate significantly. Hence, this method does not provide any sensitivity to gas content in the region of most interest. In addition, the variability of the energy efficient coatings on the glass will affect the thermal conductance and this effect might not be easily determined.
It is also possible to use acoustic techniques for measuring gas content. One acoustical method involves considering the unit as a resonant cavity and establishing a standing acoustic wave within the unit. The frequency and/or wavelength of the wave would be indicative of the gas content. However, it has been found that there are too many vibration modes, other than the fundamental cavity mode, for this method to yield useful results. Another acoustic technique involves measuring the "time-of-flight" of a sound pulse passed through the unit. While it has been found that a relatively high precision of about 2 percent can be achieved for heavy gases such as SF6, precision is only about 20 percent for light gases such as argon. Clearly, known acoustical techniques are not satisfactory for light gases like argon which is the most used gas.
There is a need for an inexpensive, fast and reliable method for non-intrusively determining the gas content of an insulating glass window unit, i.e. without destroying the integrity of the seal.